The invention relates to a heat exchanger comprising a plurality of flat tubes for heat exchange between a first fluid inside said tubes and a second fluid flowing outside of said tubes, a pair of hollow headers connected to the ends of the flat tubes, an inlet and an outlet being provided in the headers for introducing the first fluid into the tubes and discharging it therefrom, each header being composed of at least two parallel tubes with circular cross-sections two adjacent tubes having common wall portions and all tubes of each header constituting a substantially flat array of tubes.
Such a heat exchanger is known from EP-A-0 608 439.
In conventional heat exchangers, such as e.g. disclosed in EP-A-0 359 358, the header consists of a tube with circular cross-section. These tubes have been provided with holes with a shape corresponding to the cross-section of the heat transfer tubes so as to accept the tube ends. This design proves to be very satisfactory with the traditional pressures used in this type of heat exchanger. Commonly at the low pressure side a pressure of 2,5-6 bar has been used, whereas at the high pressure side pressures between 15 and 30 bar are used. With the introduction of higher pressures, the wall thickness of the header has to be increased. This is especially true for heat exchangers using CO.sub.2 at high pressure, where the low pressure is between 35-80 bar and the high pressure between 80 and 170 bar.
This increase in size of the headers has resulted in heat exchangers with large size and weight, which constitutes especially a disadvantage in heat exchanger to be used in mobile equipment such as passenger cars and the like.
The problem with respect to the strength of the header has been overcome by constructing the header as disclosed in EP-A-0 608 439.
In this header a number of parallel tubes has been provided each communicating with a number of heat exchanging tubes. A parallel flow is occurring between the different tubes of the header and the different heat exchanging tubes. A disadvantage of this system is that the pressure drops and therefor the flow patterns in the different available flow paths are all different. This leads to additional losses in pressure and irregularities in the flow which negatively influences the heat exchange.